In a certain conventional gas turbine engine, an inner shaft supporting a low pressure compressor and a low pressure turbine and an outer shaft supporting a high pressure compressor and a high pressure turbine typically consist of hollow shafts which are coaxially nested with each other. Such a gas turbine engine is disclosed in the U.S. Pat. No. 5,862,666. According to the conventional gas turbine engine disclosed in this U.S. patent, the outer and inner shafts are supported by separate bearings at their front and rear ends, and each bearing is lubricated by a forced lubrication system which blows lubricating oil fed by a pump to the bearing.
In such a forced lubricating system, to positively prevent the lubricating oil from leaking out of the bearing boxes provided in the front and rear ends of the outer and inner shafts, high pressure air drawn from the compressor and whose pressure and flow rate are controlled by using an orifice or the like is conducted to the exterior of the oil seals of the bearing boxes as seal air to keep the interior of the bearing boxes at a lower pressure than the exterior thereof. Also, a part of the high pressure air is used for cooling the turbine rotor.
The air (secondary air) for cooling and sealing can be supplied by bleeding an intermediate stage or final stage of the compressor (CDP) depending on the user of the secondary air and required pressure. Alternatively, the bleed air for sucking a boundary layer may be used for this purpose. In case of a centrifugal compressor, air may be bled by directing the flow issuing from the outlet of an impeller into a gap between the back side of the impeller and casing.
However, the gas turbine engines that supply air for cooling and sealing as described above have the following problems.    (1) Because the high-pressure, high-temperature gas (secondary air) produced by the compressor is used for purposes other than providing a thrust, the overall efficiency of the engine is reduced.    (2) Because the pressure of the air compressed by the compressor is excessively high, the air needs to be reduced in pressure by using such means as an orifice, and this means a waste of usefully energy.    (3) When the high-temperature, high-pressure seal air is introduced into the bearing box, it causes the temperature of the lubricating oil to rise. This prevents adequate cooling of the lubricating oil, undesirably heats such components as the casing typically made of aluminum alloy and gears typically made of steel, and causes premature degradation of the lubricating oil. This creates the need for adding an oil cooler of a relatively large size and increasing the amount of the lubricating oil.    (4) High-temperature compressed air is not effective in cooling a turbine so that a correspondingly large amount of cooling air is required to be bled.    (5) When an orifice controls the pressure and flow rate of the seal air so as to ensure a required seal pressure at an idle rotational speed, the pressure of the seal air tends to be excessive at a normal operational rotational speed, and a large loss is incurred owing to an excessive bleeding.    (6) In case of a large compressor including a large number of stages, it is possible to bleed air of an appropriate temperature and pressure from an intermediate stage and minimize the loss by avoiding an excessive pressure. However, in case of small engines, in particular those using a centrifugal compressor, each stage compresses air by such a large factor that it is difficult to bleed air that has an appropriate pressure and temperature.